This invention relates to spectrometers of the internal reflection type and, more particularly, to multiple internal reflection spectrometers utilizing crystals having opposed internally reflecting surfaces.
Multiple internal reflection spectrometry, also known as frustrated multiple internal reflection spectrometry or attenuated total reflection spectrometry (ATR), is based on the phenomenon that a light wave reflected from a totally internally reflecting interface is attenuated by a sample positioned beyond the interface. The light, which may include radiation within the ultraviolet, visible, or infrared ranges is attenuated at wavelengths specific to the sample.
One prior internal reflection apparatus is shown in FIG. 1. An ATR crystal 12 having a rectangular cross section is positioned in a sample chamber 14 by O-ring seals 16 and 18. A sample fluid enters the sample chamber 14 through an inlet tube 20 and leaves the chamber through an outlet tube 22. The internal reflectance crystal is illuminated by a light source through a slit 24. The slit width matches the width of the ATR crystal end face 28 and, ideally, the end face 28 is placed adjacent the slit. However, due to physical restraints of the spectrometer design, a light pipe 26 is provided to direct the beam of light through the entrance end surface 28 of the internal reflectance crystal. The beam strikes the entrance surface substantially perpendicularly; thus the beam is transmitted into the crystal. The beam of light then strikes the internally reflecting surface 30 at an angle of incidence greater than the critical angle and is reflected to the opposed surface 32. The angle of incidence of the light beam striking the surface 32 is also greater than the critical angle. Thus, the light is reflected back and forth between the opposed internally reflecting surfaces 30 and 32 toward an exit end surface 34 bevelled with respect to the reflecting surfaces. The beam of light strikes the end surface 34 substantially perpendicularly and thus passes on to a detector (not shown). The frequency spectrum of the detected light beam identifies the sample adjacent the totally internally reflecting interfaces and allows its concentration to be determined.
The spectrometer shown in FIG. 1 presents several practical problems. For one, a tight seal is difficult with an O-ring surrounding a crystal of rectangular cross section. The right angles of the crystal tend to break the seal. This problem can be minimized by rounding the edges of the crystal or by use of seals as shown in FIGS. 2 and 3. O-rings 36 and 38 are positioned flush against the opposed internally reflecting surfaces of an ATR crystal 40 to define two separate sample chambers.
The utility of spectrometers using internal reflection sampling has been limited by the severe energy losses associated with the ATR crystal. In prior configurations an image of the spectrometer slit is formed on the entrance face of the ATR crystal, filling its width, and the beam diverges inside the crystal so that either the top and bottom edge surfaces 42 and 44 are contacted by the beam or the emerging beam is considerably higher than the entering one. Both situations lead to energy loss. This is particularly true for filter spectrometers with "fast" optical systems. For example, the beam emerging from the slit of one filter spectrometer is a rapidly divergent beam, about F/1.5. There are further energy losses due to light striking the O-ring seals, particularly where the seals are as shown in FIGS. 2 and 3.
The sealing of FIGS. 2 and 3 is used successfully with ATR systems having a less divergent beam, such as about F/6, passed through the center portion of a tall ATR crystal, for example, a crystal 20 millimeters (mm) high. With such systems, the beam of light does not diverge rapidly enough to completely fill the crystal; thus losses at the upper and lower edges and at the O-ring seals are avoided. However, the tall exiting image has a low energy density and cannot be efficiently directed to the detector. And a large aperture at the light source is often vital in providing a high signal to noise ratio. Hence, the less divergent beam is not always feasible.
An object of this invention is to provide an illuminating system in an internal reflection spectrometer which permits the use of a wide aperture light source and a large number of internal reflections while avoiding the losses which generally result from reflections from the upper and lower crystal edge surfaces and from O-rings flush against the internally reflecting surfaces, or from an oversize exit beam which cannot be efficiently transferred to a small detector.